X/r and MVA Info From SDG&E

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stuart6gw

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Has anyone had success obtaining x/r and MVA values from San Diego Gas & Electric for their network or for a specific location/area of their network?
 

ron

Senior Member
I haven't worked with that utility, but they are all difficult to get information from in that level of detail. Especially when I ask for those values in the best and worst cases, so I can do short circuit calculations for equipment AIC and withstand specification and the other values for arc flash calculations, they stall and struggle even more.

Stay after them, they will cough up the information.
 

stuart6gw

Member
x/r and MVA Info From SDG&E

x/r and MVA Info From SDG&E

Well, I persisted in my attempts to get this info from SDG&E. No success. I made several attempts to speak with one of their engineers. I got no response. Their engineering department finally forwarded my request to one of their senior project managers. He called to explain that in all his time with SDG&E, he has never had a request for this information. I explained why I needed the information. He admitted that he really didn't understand this level of the technical side of their network, but he just couldn't help me.

Since their generator sources are typically in Texas and Arizona, and my point of interest is in San Diego, California, I could use some nominal range for x/r that applies to power supplied over long transmission lines, and an assumed value for MVA. Any input on this?
 

ron

Senior Member
Is this study for short circuit ratings on the equipment or arc flash.

If for short circuit, try to find the size of the closest upstream transformer from your service and use the infinite primary calculated through to the secondary. 8 is what I use if I have to take a guess for x/r. That will give you a conservative number to use.

If you are calculating for arc flash, you will need to be creative and make that short circuit available value lower.
 

Besoeker

Senior Member
Location
UK
Is this study for short circuit ratings on the equipment or arc flash.

If for short circuit, try to find the size of the closest upstream transformer from your service and use the infinite primary calculated through to the secondary. 8 is what I use if I have to take a guess for x/r. That will give you a conservative number to use.

If you are calculating for arc flash, you will need to be creative and make that short circuit available value lower.
Coincidentally or otherwise, that's something I've just done for our test bay. We have a utility company transformer in our yard and, like others here, It proved very difficult to get information from them. Eventually I got the Z and estimated x and R from that.
I took that and the characteristics of the cable from our main incomer board to our test bay board.
The effect of including the cable was to drop the available fault current below the 10kA figure.
Had it not, I was going to get a three phase choke made to do that.
 

stuart6gw

Member
x/r and MVA Info From SDG&E

x/r and MVA Info From SDG&E

Thank you for your responses. I am doing a short circuit analysis. I have seen tables recommending x/r values of 2 to 12 for power supplied over long transmission lines. So an x/r value of 8 would be a good conservative approach Ron.

Also, this study is for an existing electrical installation that I have been hired to perform inspection and maintenance for. I have done the calcs for this facility using the simplistic method of "f" factor and "M" multiplier. All the equipment ratings are fine. But I would bet that, using a more realistic method that takes into account asymmetrical short circuit currents, that the 10k rated CB's in at least one of the panels will "flunk". Now, the City of San Diego electrical inspector would require SSC calcs if this were a new installation, but they would accept the simplistic method I described above. So, as far as the AHJ is concerned all would be fine. The least expensive "fix" in this scenario would be to replace the panel feeder breaker with one that is series rated for the panel breakers. But the only question my client will have is, "Is it safe as it is." In light of the lack of input from the utility, my report would recommend that the breaker be replaced. Thoughts?
 

rbalex

Moderator
Staff member
Location
Mission Viejo, CA
Occupation
Professional Electrical Engineer
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stuart6gw

Member
x/r and MVA Info From SDG&E

x/r and MVA Info From SDG&E

Thanks for the suggestions Bob. I mistakenly referred to the SDG&E person I've been trying to work with as a project manager. He is actually their senior project planner. And as I said, he says he has never had anyone inquire about this type of information before. He talked with his engineers, and apparently, they can't help me either.

I am familiar with page 006 of the standard you referred to. I tells me that the available SSC will never exceed 42,000 amps. Unfortunately, that worst case number will be expensive for my clients also. The available SSC at the xfmr secondary is only 24.5k.
 

kingpb

Senior Member
They say it will never exceed 42kA because that is the typical design limit of the equipment in the substation.

Every planning dept I've had to work with has this info., just a matter of getting to the right person.
 
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